Asynchronous method for sampling signals in metal detectors

ABSTRACT

This invention is related to the method providing computation of the signal frequency components in an acceptable accuracy in contravention of the shifts in the phase and the magnitude information caused by asynchronous sampling of the signals in the process of asynchronous sampling of metal detectors wherein the received signal by the receiver unit ( 4 ) divided into time intervals, say timing values those are far shorter than the sampling period and correspond to nearest probable sampling of the ADC ( 6 ); providing the computation of the sine and cosine coefficients or exponents of time constant coefficients of the said timing value from previously located or dynamically generated coefficient table; resulting the elimination of the requirement of synchronous sampling and the requirement of the signal period is multiple of the sampling period.

TECHNICAL FIELD

This invention is related to a metal detector and the method of thatmetal detector which provides asynchronous sampling of voltage signalsconverted from a receiver unit using an analog to digital converter(ADC), originally generated by a single-frequency, multi-frequency ortime-domain magnetic field composed by a transmitter unit, thenproviding calculation of signal components in acceptable accuracy incontravention of the shifts in the phase/time and magnitude informationcaused by asynchronous sampling of the signals.

PRIOR ART

Metal detectors are expected to detect the targets that are beingsearched in a fast and reliable manner. The ferromagnetic components ofsoil and other metallic objects can nevertheless cause false alarms.

While the false alarm rate is higher in single frequency metaldetectors, this rate can be reduced in multi frequency metal detectors.Thanks to the multiple frequencies contained in the transmitted signalsof multi-frequency metal detectors, the discriminating ability inreturning signal increases and hence, rate of the false alarmsdecreases. The time-domain metal detectors have similar aspects with thesingle frequency and multi frequency relation when they apply patternsinstead of a single rectangular pulse.

In the state of the art, some metal detectors operate at a singlefrequency but permit the user to select one of the frequenciespreviously set. The frequency selection is advantageous fordiscriminating dissimilar metallic targets in soils with variousferromagnetic properties.

Nevertheless, the options of frequency remain quite yet to be sufficientin these detectors.

It is required in modern metal detectors that the analog signals thosereceived from receiver unit to be sampled (digitized) in order to beprocessed and analyzed. The digitized said signal can be defined as thesampled signal obtained from multiple number of points of the analogsignal in time.

Within this context, the first step in analog to digital conversionprocess is electrically acquiring the time variant magnetic fieldgenerated by the transmitter. This process takes place in switchingcomponents.

In the existing metal detectors, the switching process of the switchingcomponent is typically accomplished by a controller by synchronoussampling.

The voltage source that switched by the switching component istransferred to the transmitter unit and a magnetic field is generatedproportional to the current by means of the transmitter coil. Thismagnetic field, also the target is located within, generates Eddycurrents and causes regeneration of a magnetic field by the target. Themagnetic field that has been regenerated by the target induces a voltageon the receiver coil and the signal reaches to the synchronousintegrator unit through the receiver unit. The controller providestaking of the definite integral of the waveform of the signal inpredetermined intervals by means of the control signals applied to thesynchronous integrator unit. The definite integral of the signal isbeing digitized in sampling unit, say, the ADC (analog to digitalconverter). The controller computes in-phase and quadrature componentsof the sampled signal by timing synchronous to that of the transmitter,in corresponding phases and vectors consisting of two or moredimensions.

Another alternative of taking the definite integral by utilizing analogcircuit components is the sample and hold (S/H) method. Using sample andhold method, the data is being computed to vectors by negligibly shortintervals synchronously to the transmitter unit.

Within this operation, the ADC samples the signal in equal and repeatingintervals (as t₁, t₂, t₃, t₄, t₅, t₆, t₇ . . . ). In case the fullperiod of the signals is expressed as 360 degrees; the ADC, for instancein (FIG. 2), acquires the signal at ti corresponding to 0 degree, at t₂corresponding to 120 degrees, at t₃ corresponding to 240 degrees. In thesecond period of the signal these timings will correspond exactly to t₄,t₅, t₆ and be 0 degree again at t₇. The sampling frequency of the ADC isrequired to be multiple of the frequency of the received signal in orderfor the vector to be calculated by the end of the operation. The morethe speed of ADC rated to signal frequency the more samples can begathered.

The computed values will be erroneous or shifted in phase or magnitudeif the sampling intervals of the ADC are distinct from the regularsampling periods. Resultantly, the in-phase and quadrature components ofthe signal will be calculated discrepantly for each period.

The properties of sampling such as, the resolution of the ADC, thejitters in timing of the sampling are important for the performance ofthe method. These requirements limit the range of the ADC's selectablefor suitable resolution and speed.

In the state of the art; the patent titled “Metal Detector” withreference U.S. Pat. No. 7,579,839 describes a metal detector whichtransmits a rectangular wave pattern and samples the signal in thereceiver unit using “synchronous demodulators”, providing the result byanalyzing the samples. Synchronous demodulation; denotes that the analogsignal voltage from the receiver unit is being processed synchronouslyto the transmitter.

The synchronous integrator; that has been described in the patentdocument with reference U.S. Pat. No. 7,579,839, obtains the definiteintegral in the electronic form by accumulating the electric charge on acapacitor by switching the voltage over that, in other words by simplyaveraging the voltage in a time interval within the period. Thanks tothe method, the sampling process can be performed using a slower ADC byacquiring the averaged voltage on a capacitor. The requirement of thefrequency of sampling dependency to the frequency of the signal is notessential because the synchronization of the signal to be sampled isprovided by this synchronous demodulator. This solution causesadditional electronic components to be used and as a result, an increasein the cost. The electronic components used, cause variation inperformance of parameters depending to the temperature and aging,resultingly a variation between produced devices. The patent documentreferenced by U.S. Pat. No. 7,579,839 encloses the method of operationmay subject to synchronization by said synchronized sampling, althoughit is not seemed to be directly related to “demodulation” concept.

The intervals of sampling of the signal by the ADC are predeterminedwithin the period in the process of digitizing which is carried out by adigital synchronous receiver. Hence, it is also predetermined which timecorresponds to each phase or angle. The values of sines and cosines ofpredetermined angles are usually stored in a table as coefficients inorder to calculate the vectors in a faster manner. Thereafter, eachsample is multiplied by the corresponding sine and cosine coefficientsto separate it to the vectoral components. This method of operation canbe expressed as a calculation method known as the Single Bin DiscreetFourier Transform (Single Bin DFT) below.

${V_{X}\lbrack f\rbrack} = {{\sum\limits_{n = 0}^{N - 1}{{{v\lbrack n\rbrack} \cdot \cos}\;\frac{2\pi\;{nf}}{f_{s}}\mspace{14mu}{ve}\mspace{14mu}{V_{Y}\lbrack f\rbrack}}} = {\sum\limits_{n = 0}^{N - 1}{{{v\lbrack n\rbrack} \cdot \sin}\;\frac{2\pi\;{nf}}{f_{s}}}}}$

In the expression above; “V_(X)[f] and V_(Y)[f]” are the components ofthe vector in X and Y axis at specified frequency, “N” is the totalnumber of the samples acquired, “v[n]” is the sample acquired atsequence “n” in a period, “f” is the frequency of signal and “f_(s)” isthe sampling frequency. The number N required to be relevant to therelation between signal frequency “f” and the sampling frequency “f”.For instance; the number N can be selected as N=∥fs/∥ in calculating theDFT after the period is completed. Here, the operator ∥ ∥ represents theround to the closest integer number. The sampling frequency is requiredto be multiple of the signal frequency and also all samples are requiredto cover the period of the lowest frequency component. Thetransformation either can be calculated separately for each frequency orcalculated for all frequencies at once using a common algorithm, i.e. byFast Fourier Transform (FFT) method.

The above formula; sin

${{\sin\left\lbrack \frac{2\;\pi\;{nf}}{f_{s}} \right\rbrack}\mspace{14mu}{ve}\mspace{14mu}{\cos\left\lbrack \frac{2\;\pi\;{nf}}{f_{s}} \right\rbrack}},$

values are coefficients for the DFT calculation.

These coefficients are located in a table aforementioned, can also becalled as look-up table that is usually located in the memory of orattached to the computing unit in the design For example; the look-uptable will contain 3 sine cosine pairs of values if the frequencies aref=1 kHz and fs=3 kHz. Similarly, the look-up table will contain 8 pairsof coefficients if the frequencies are f=1 kHz ve fs=8 kHz.

In case fs is the multiple off, the first sample of the signal will beacquired right at the same point of the repeating signal. Therefore;equal number of samples are acquired for each period. Within thiscontext; the sine and cosine coefficients can be always selected fromthe predefined table in a correct form. The transmitter and receiverunits of a system operating in this manner can be called “synchronous”.

It cannot be expected that acquiring samples in the same number and fromthe same points during the each period of signal in case offs is not themultiple of the f′. Within this context, sine and cosine coefficientswill be required at distinct points in the period. In this case; thephase information will shift at the end of each period. The transmitterand receiver units can be said “asynchronous” state in a systemoperating this way.

The phase information of a synchronous system is being measuredrepeatedly, without any alteration requirement of the coefficients.Despite, an asynchronous system can operate any frequency conforming the“Nyquist Criterion”, but the phase information cannot be measured insame way because it varies for each period of the signal.

Short Description of the Invention

The method related to the invention provides calculation of the phase byan acceptable accuracy using asynchronous sampling in contravention ofshifts in the phase and magnitude information of the acquired signal. Inorder the computations can be accomplished, dynamic coefficients areused, providing signal phase information tracked much faster than thesampling speed, despite using look-up tables containing the sine andcosine coefficients according to a sampling frequency.

The objective of the invention is; sampling the received signal from thereceiver unit using a fast and high-resolution ADC asynchronously andcalculation of the in-phase and quadrature components with minimal errorin a metal detector that generates the magnetic field in a form ofsinusoidal or rectangular patterned waveform or a pulse train.

This invention eliminates the necessity of the signal period to bemultiple of the sampling period of the ADC by means of a method, candevise asynchronous sampling. Thereby, the metal detector usingasynchronous receiver can generate signal at any required frequency atthe transmitter unit.

Thanks to the invention, the incoming signal can be analyzed in moredetail by comparison with the synchronous sampling.

Another advantage of the invention is the elimination of additionalelectronic components, that may be used in case to provide asynchronization, resulting waste of power and electrical noise.

The invention provides opportunity of selection of the ADC from widerrange in specifications for its resolution and speed. Within thiscontext the advantage is the substitution of Sigma-Delta ADC's those arein a lower cost and with numerous options in resolutions and speedrather than the synchronous SAR ADC's (Successive Approximation RegisterAnalog to Digital Converter).

The present invention is described in more detail by way of references,with reference to the figures and graphics listed below:

DESCRIPTION OF THE FIGURES

FIG. 1: Sample block diagram of a digital metal detector.

FIG. 2: The graphical representation of the sample timing of thereceived signal. The (t_(n)) timings point to the samples synchronouslyacquired whereas (t′_(n)) point to the timing of asynchronous samples.The numbers 17, 18, 19, 20 . . . 36 depict the counter values of thetiming.

DESCRIPTION OF REFERENCE NUMBERS

NO PART NAME 1 Transmitter Coil 2 Receiver Coil 3 Transmitter Unit 4Receiver Unit 5 Switching Component 6 ADC 7 Controller

DESCRIPTION OF THE INVENTION

This invention is related to the metal detector containing transmitterunit (3) generating a single frequency, multifrequency of pulsedmagnetic field and providing the electrical reception of the magneticfield by the receiver unit (4) which is created at the target as aresult of the field time-varying magnetic field generated by thetransmitter unit (3) wherein; the signal received by the receiver unit(4) is sampled asynchronously relative to the transmitter unit by atleast one ADC (6) (Analog to Digital Converter) and analyzed by thevectors of the sinusoidal components are obtained in this way.

Within the metal detector which the sampling is accomplishedasynchronously; the switched signal generated by the controller (7)containing the multifrequency components is applied to the switchingcomponent (5). The voltage switched by the switching component (5) isapplied to the transmitter unit (3) and a magnetic field is generated atthe transmit coil (1) or around the transmit coil (1) which isproportional to the electric current flow. The magnetic field creates acurrent flow within the target and as a result, the current flow at thetarget generates a magnetic field and induces a voltage on the receivercoil (2) to be transferred to the ADC (6) through the receiver unit (4).The in-phase and quadrature components of the signal that is sampledasynchronously by the ADC (6) are computed by the controller (7). Sameprocess is valid for the time-domain metal detectors such as pulseinduction detectors were the phase components replaced with theexponential time constants.

The period of the sampled signal is not the multiple of the samplingperiod utilizing asynchronous digitization by the ADC (6). Accordingly,it is not predictable to determine the point that ADC (6) will samplethe signal. Therefore, it is not useful that a predetermined sine andcosine table located in the controller (7) memory. Conversely, thegreatest advantage of the asynchronous configuration is providing anopportunity to use a faster and high resolution ADC (6) at a bearablecost. Albeit, it is required to predetermine the time intervals betweenthe asynchronous sampling and that of synchronized version in order tothe asynchronous configuration can be applied (in order to be applied ofthe asynchronous configuration).

This invention provides the compensation of the probable shifts inmeasurements resulting from asynchronous sampling by means of thepredetermination of the time intervals between asynchronous timing ofthe ADC (6) and that of synchronous version.

In the asynchronous signal sampling method of the invention; thecontroller (7) divides the multifrequency signal period into equal timeintervals, based on the required resolution. The period of each saidinterval has a far less duration than sampling period and a digitalrecord is located for each interval. The timing values can be tracked bymodules such as a timer or a counter. A table is constructed as eachlocation holds the sine and cosine coefficients of the correspondingtime interval. The steps of the table correspond to the phase (angle) ofcorresponding time interval. The content of the table is depended to thefrequency to be analyzed, therefore the table content is required to beconstructed based on that frequency. The tables can be priorlyconstructed in case the frequencies are also predefined. The majordifference to the prior art is, the tables are not defined correspondingto the sampling times but rather defined to probable time intervals thatthe sampling to be done.

If the sampling frequency is fs and the time gap is defined as Δt;

Δt=1/fs

Above expression defines the time gap as the error caused by thesampling frequency. Same equation can be used to express the timingerror as the “accurate timing” concept in case of the ADC sampling speedincreases. Within this context, in the following expression;

Δt′=1/(Mfs)

“M” stands for the interval timing precision of the samples acquired bythe ADC (6) where M>1. The said timing values are used to track thephase change of the asynchronously sampled signal. Resultingly, a M*Nsize table that containing the sine and cosine coefficients, isconstructed, where N is the total number of samples within a signalperiod. The timing value can be used to index the sine and cosinecoefficients from the table in ratio of the value at the end of thesignal period. Finally, the vectoral components are obtained bymultiplying the sine and cosine coefficients by the correspondingsampled values. Within this context; the sensitivity of the result ofcalculations increases with the resolution of sampling.

The said asynchronous sampling system described by FIG. 2; the ADC (6)samples the received signal at the instants of t′₁, t′₂, t′₃, t′₄, t′₅,t′₆, t′₇ . . . with equal or unequal sampling intervals butnon-coherently to the repeating signal. In the asynchronous samplingpart of FIG. 2, the ADC (6) samples the 0 degree at t′₁, 111 degree att′₂, 221 degree at t′₃ and 332 degree at t′₄ at the first period. In thesecond period, ADC (6) samples 83 degree at t′₅, 193 degree at t′₆ and304 degree at t′₇ . As seen in the example, the ADC (6) samples thesignal asynchronously, by equal intervals but not corresponding to samedegrees in following two periods. This case results measurement of thephase of the sampled signal different from the other in each signalperiod. In the method of the invention; the phase shift is tracked bythe controller (7) and used for the computation. The controller (7)takes reference timing values while it applies the reference signal tothe switching component (5) generated using the transmitter unit (3). Inthe method of invention; the signal is divided into smaller timeintervals (Δt′). The said time intervals are depicted as 17, 18, . . .36 in FIG. 2. The reference timing values indexes the time intervals as0, 1, 2, . . . 19 and provides a calculation of the corresponding phase(angle) values. The timing values can be tracked by modules such as atimer or a counter. The phase (angle) value is calculated in eachsampling of the ADC (6) according to the reference timing value. Thanksto the referenced timing; the synchronization shift of the ADC (6)sampling to the received signal is determined as well as the phaseshifts because of the asynchronous sampling.

This invention is a method of; sampling of the signal by at least oneADC's (6) which received through a receiver unit (4), generated as aresult of the reaction of the target to the magnetic field generated bya transmitter unit (3); and utilization of a DFT based transformationtechnique consisting of the summing of the samples each multiplied bythe coefficients corresponding to the timing within the period in orderto be expressed in terms of components of phase/magnitude or timeconstants in order to obtain the information for the target presenceand/or the target kind for the metal detectors. In addition to thesynchronous methods in the state of the art, the invention providesasynchronously sampling by following steps;

-   -   Defining the nearest timing of the each acquired sample from the        table/array by using the transformation table/array which is        defined for the timing intervals that are smaller than or equal        to the sampling intervals of the ADC (6) and multiplying the        coefficients corresponding to the defined time with the acquired        samples.    -   Phase/magnitude and/or time constant components of targets are        obtained in acceptable accuracy by using the sum of the        asyncronously acquired samples, each multiplied by the        corresponding coefficient.

In the preferred embodiment; the timing values are generated by internalcounters within the processor. The size of the said sine and cosinecoefficients table and the speed of the said counters (number of thetiming values) varies with the required resolution and frequencyspectrum to be analyzed.

In the preferred embodiment, the detector is a multifrequency VLFcontinuous wave (CW) detector while all described analogy can be appliedto time-domain detectors operating by pulse induction method or anyhybrid methods by time-domain analysis with replacement of sine andcosine coefficient tables to corresponding time domain analysiscoefficients and expressing them as superposed series of exponentials oftime constants.

1. A method for obtaining information related to a target presenceand/or a target kind for metal detectors, the method comprising:sampling a signal by at least one ADC which is received through areceiver unit wherein said signal is generated as a result of a reactionof said target to a magnetic field transmitted by a transmitter unit; bya controller, utilizing a DFT based transformation technique consistingof summing of samples where each sample is multiplied by coefficientscorresponding to a timing within a period in order to be expressed interms of components of phase/magnitude or time constants for obtainingsaid information; by the controller, providing the transmitter unit totransmit a reference signal where said reference signal is divided intotime intervals where said time intervals are indexed by reference timingvalues; by the controller, providing the receiver unit to receive andADC to sample said reference signal; by the controller, detecting aphase shift for each sampling of ADC according to a reference timingvalue; by the controller, updating a transformation table, thetransformation table defined for the timing intervals that are smallerthan or equal to sampling intervals of the ADC and multiplyingcoefficients corresponding to a defined time with acquired samples, bythe controller, providing transmitter unit to transmit a signal fordetection, by the controller, providing receiver unit to receive and ADCto sample said signal, by the controller, defining nearest timing ofeach acquired sample from a table/array by using said the transformationtable/array and determining corresponding coefficients for each sample;and by the controller, obtaining phase/magnitude and/or time constantcomponents of targets by using the sum of the asynchronously acquiredsamples, each multiplied by the corresponding coefficient.
 2. (canceled)3. The method according to claim 1, wherein the said metal detectoroperates any of single frequency, multifrequency or time-domain methods.4. (canceled)
 5. The method according to the claim 1, wherein a table onan internal or external memory of the processor is used for locating thesine and cosine coefficients corresponding to each timing value ordetermined by calculation, dynamically.
 6. The method according to theclaim 5, wherein the time constants coefficients are located in thetable in the memory when using time-domain, say pulse induction method.7. The method according to the claim 1, wherein the computing, timing orsignal conversion processes are accomplished by at least onemicrocontroller, at least a digital signal processor (DSP) and/or atleast one FPGA.